The trend in modern structural design has been to utilize higher strength materials to meet higher design stress goals while reducing overall material costs. However, these trends have been accompanied by an increased risk of structural failure resulting from an inherent loss of ductility and, in many cases, a reduction in design safety factors. Residual stresses inadvertently introduced during manufacturing and fabrication processes such as forming and welding can no longer be ignored; such initial stresses must be carefully determined and considered to establish safe loadcarrying capability of modern structures. This is because residual and applied stresses may combine to exceed design allowables. Therefore, in recent times there is greater interest in the methods for measuring both applied and residual stresses, especially nondestructive methods of residual stress measurement.
In metals, a widely accepted nondestructive residual stress measurement method is x-ray diffraction, but it has several practical limitations and can only measure stress within a few thousandths of an inch of the surface. More recently, because of the ability of acoustic energy to penetrate much deeper into most metallic materials, there has been considerable development activity associated with the ultrasonic method for measuring bulk residual stresses, especially the shear wave birefringence technique. This ultrasonic method, based on the acoustoelastic effect, has shown good potential for use in measuring bulk residual stresses in homogeneous and isotropic materials. However, practical application of ultrasonics has been severely limited by material variables such as preferred grain orientation, composition, prior magnetic history (for example in steels) and inhomogeneity which, in some cases, can totally mask the acoustoelastic effect.
This invention relates to a new ultrasonic method for measuring stresses in ferromagnetic materials using the magnetoelastic effect. One version of this method utilizes the stress dependence of the change in ultrasonic velocity induced by an externally applied magnetic field. As a result of the investigations underlying the present invention, it is believed that the velocity changes of ultrasonic shear waves induced by externally applied magnetic fields are characteristically dependent on the relative orientation of the stress, the shear wave polarization, and the magnetic field, as well as on the magnitude and sign of the stress, i.e., tensile or compressive. Importantly, it appears that the magnetically induced velocity change (MIVC) is insensitive to the elastic anisotropy caused by preferred grain orientation. Further, ensuing investigations suggest that MIVC of ultrasonic longitudinal and surface waves is also characteristically dependent on the relative orientations of the stress, the polarization and the propagation direction of the ultrasonic wave, and the magnetic field, as well as on the magnitude and sign of the stress.
An example of the characteristic stress dependence of MIVC is shown in FIGS. 1 and 2 which were obtained with 4.5 MHz ultrasonic shear waves in A-36 steel specimens under uniaxial tensile stresses. FIG. 1A is for the case where the shear wave polarization (S), the magnetic field (H) and the stress (T) are all parallel (abbreviated S.parallel.H.parallel.T). Similarly, FIG. 1B is for the case where S and H are parallel but the two are perpendicular to T (abbreviated S.parallel.H.perp.T). In the graphs, the magnetically induced velocity change is .DELTA.V. .DELTA.V is defined as .DELTA.V=V.sub.H -V.sub.H where V.sub.H is the velocity at field H and V.sub.O is the velocity at H=O. Both graphs are normallized for V.sub.O as observed on the abscissa. The MIVC curves in FIG. 1A and 1B are identical at T=O. However, as the tensile stress increases, the curves in FIG. 1B exhibit distinctly different behavior from those in FIG. 1A. The decrease in the magnetically induced velocity changes with stress is much larger for S.parallel.H.perp.T than for S.parallel.H.parallel.T, and the curve at T=20 KSI shows a characteristic minimum for S.parallel.H.perp.T. The characteristic dependence of MIVC on the stress magnitude such as shown in FIG. 1B is the basis for determining the magnitude of an unknown tensile stress.
The MIVC curves in FIGS. 1A and 1B are redrawn in FIGS. 2A and 2B to show their dependence on the relative orientations of S, H and T. FIGS. 2A and 2B show the MIVC curves for fixed stresses of 10 KSI and 20 KSI, respectively. As shown in FIGS. 2A and 2B, the velocity change induced for a given H value is largest for S.parallel.H.parallel.T and smallest for S.parallel.H.perp.T. When T is at an angle to S.parallel.H between 0.degree. to 90.degree., the corresponding velocity change value lies between the two extreme values obtained for S.parallel.H.parallel.T and S.parallel.H.perp.T, respectively. Such characteristic relative orientation dependence of MIVC makes the determination of the direction of a stress possible. Changes in MIVC of ultrasonic shear waves as a function of both uniaxial tensile and compressive stresses and relative orientations of T, S and H in A-36 steel can be found in detail in references H. Kwun, "Stress Measurement in Ferromagnetic Material", Final Report, SwRI Project 15-9297, August (1982); H. Kwun and C. M. Teller, "Tensile Stress Dependence of Magnetically Induced Ultrasonic Shear Wave Velocity Change in Polycrystalline A-36 Steel", Appl. Phys. Lett. 41, 144 (1982).
In addition to the stress, the characteristics of MIVC are in general a function of the magnitude and the sign, i.e., positive or negative, of the magnetostriction coefficient of a material and the frequency and the wave mode, i.e., longitudinal, shear or surface, of an ultrasonic wave employed. Therefore, to determine an unknown stress in a given structural member by the present invention, calibration MIVC curves should be obtained first over a range of applied stresses using the same nominal alloy material; the alloy need not be identical, particularly in terms of preferred orientation or elastic anisotropy on calibrating for the same ultrasonic wave mode and frequency as that to be used in a test. Basically, an unknown stress in a structural member is determined by comparing measured MIVC curve(s) in the structural member under testing with a set of calibration MIVC curves obtained in the specimen material.
To determine bulk, surface as well as gradient of stresses, MIVC curves of an ultrasonic shear or longitudinal wave as well as those of ultrasonic surface waves at several different frequencies are required. Usually the penetration depth of a surface wave is approximately equal to the wavelength of the wave; stresses at different depths below the surface of a material can be sensed using surface waves of different frequencies.
The method proposed in this invention utilizes a C-shaped magnet having a pair of pole faces which are positioned adjacent to a structural member under testing. This magnet forms magnetic field lines in the test specimen tangential to the surface of the specimen. Appropriate ultrasonic transducer(s) are attached to the specimen. For the case of ultrasonic surface wave, two transducers are required; one is used as the transmitter and the other as the receiver of the surface wave. Ultrasonic transducers are coupled to the member using an appropriate couplant, such as oil, water or a shear wave couplant having a high viscosity. The applied magnetic field is measured using a tangential Hall effect probe placed adjacent to the ultrasonic transducer, and the change in velocity (more specifically the change in the time-of-flight) of the ultrasonic wave as a function of magnetic field can then be obtained. For accurately measuring the change in velocity, either a pulse-echo overlapping technique or a relative phase (to a reference rf wave) measuring technique can be used. One such technique is described in R. J. Blume, "Instrument for Continuous High Resolution Measurement of Changes in the Velocity of Ultrasound", Rev. Sci. Instrum. 34, 1400 (1963). In the case of surface waves, the use of a relative phase measuring technique is the only choice since there is only one received signal and there are no echoes to overlap.
It has been found that magnetic hysteresis effect on the magnetically induced velocity changes is negligible in most cases. It appears also that there is no difference by reversing the magnetic field.
The procedure set forth hereinbelow particularly finds ready application in testing structural components in an assembled framework and the like. Consider the situation where a support framework is fabricated for heavy equipment. It is possible to use the procedure described herein to test the installed frame components before or after manufacture to determine the stress in those components. After installation, there is no low cost test; testing cannot be achieved through the application of strain gauges because strain gauges work only on incremental change arising as a result of strain. They can only be utilized if the strain gauge is attached before a strain occurs in the specimen of interest. One approach is x-ray testing. It is particularly limited. This novel procedure enables testing without having access to the unstressed frame member before stress is placed on the frame member. Alternatively, the frame member can be tested solely for residual stress before installation. Moreover, the procedure disclosed herein can be quickly implemented and the equipment easily affixed on and removed from the specimen undergoing test. Again, assume that a framework for supporting heavy equipment is to be tested. In this context, the frame members may terminate in a lattice work at suitable fasteners, and access to the frame members is easily achieved. This apparatus enables the testing of stress (both residual and loading stress) in the frame members at intermediate points without access to the ends of the frame members.
Other advantages depend on a variety of circumstances in application of this procedure. It is therefore summarized as a procedure for determining stress in a ferromagnetic member utilizing an applied magnetic field to the member and measuring the change of velocity in ultrasonic waves transmitted through the test specimen.